Divalent metal stannate products are being increasingly used as flame-retardant additives in polymer formulations. They generally exist in two forms of formula MSn(OH)6, and MSnO3, the latter being readily formed by heating the former to drive off water, and also in a third form of formula M2SnO4, where M represents the divalent metal. Thus for example zinc hydroxy stannate (ZnSn(OH)6) and zinc stannate (ZnSnO3) have been marketed since 1986 under the trademarks Flamtard H and Flamtard S. Flamtard S is made by heating Flamtard H at 400xc2x0 C. for a sufficient time to drive off the combined water. Flamtard S commands a premium pricexe2x80x94but it has superior thermal stability and is recommended for high temperature formulations.
This invention results from the discovery that an intermediate product has properties which are in some respects better than either Flamtard H or Flamtard S. This discovery was surprising. It was expected that an intermediate product would have intermediate properties.
The invention provides in one aspect a divalent metal stannate product which has a loss on ignition of 2-17% by weight, said product in packaged form for use as a flame-retardant additive. Preferred divalent metals, on account of the economic importance of their stannates are Zn, Mg, Ca, Ba and Bi; most preferred are Bi and particularly Zn.
In compounds where the atomic ratio of the divalent metal to tin is 1, a divalent metal hydroxy stannate has the formula MSn(OH)6. On heating at 400xc2x0 C., this product is converted to the corresponding divalent metal stannate MSnO3 and gives off 3 moles of water per mole of metal stannate. The term xe2x80x9cmetal stannate productxe2x80x9d is herein used to cover the metal stannate and partially hydrated compounds not including the fully hydrated divalent metal hydroxy stannate.
The loss on ignition of a compound is determined by first drying the compound at 110xc2x0 C. to constant weight, then heating the compound at 1000xc2x0 C. to constant weight, and noting the percentage difference between the two. The loss on ignition of divalent metal hydroxy stannates depends on the atomic weight of the divalent metal. For example, the loss is theoretically 18.9% for zinc hydroxy stannate and 12.6% for bismuth hydroxy stannate. The loss on ignition of a divalent metal stannate is in principle zero but may in practice be up to 1.5%.
The divalent metal stannate products of the present invention are characterised by having a loss on ignition of at least 2% preferably at least 4%; but less than the corresponding divalent metal hydroxy stannate. For zinc stannate products the loss on ignition is preferably 2-17% e.g. 4-15%. For bismuth stannate products the loss on ignition is preferably 2-11% e.g. 4-9%. Preferably the divalent metal stannate product has a formula MSnO3.xH2O where x is 0.4-2.6 e.g. 0.8-2.2.
These divalent metal stannate products may be made by heating a corresponding divalent metal hydroxy stannate under conditions to drive off a desired proportion of the combined water present. Suitable heating temperatures are 200-350xc2x0 C. It may be convenient to use the same kind of rotary kiln as is currently used for converting Flamtard H to Flamtard S, e.g. for 2-40 minutes. Of course, that process produces a transient partially dehydrated zinc stannate product, but that transient product is in commercial production never recovered and packaged for use as a flame-retardant additive.
Or the metal hydroxy stannate may be heated in a spin flash dryer, preferably at a temperature from 130xc2x0 C. to 400xc2x0 C. Or the metal hydroxy stannate may be heated in a static oven e.g. for 30 minutes to 5 hours. Different heating regimes give rise to slightly different products which may be optimum for different purposes. In general, the heated intermediate product of this invention is white (like Flamtard H) rather than pale yellow (like Flamtard S or a mixture of H and S).
Alternatively the divalent metal stannate products of this invention may be made simply by mixing together the corresponding divalent metal hydroxy stannate and divalent metal stannate in suitable proportion. For example, 90xe2x88x9210 wt % of Flamtard S may be mixed with 10-90 wt % of Flamtard H. This simple mixture has properties which, while generally not as good as those of the heated products, are nevertheless surprisingly superior to both the Flamtard H and the Flamtard S when used separately.
In another aspect the invention provides a polymer formulation containing an effective concentration of a flame-retardant additive which is a divalent metal stannate product as defined, either alone or in admixture with a divalent metal borate. For example, such a mixture may consist of 90% to 10% by weight of the divalent metal stannate product and correspondingly 10% to 90% by weight of the divalent metal borate. Zinc hydroxy stannate and zinc stannate may be added to the polymer formulation either separately or together as a pre-mixture.
Polymer formulations may be those in which zinc (or other metal) hydroxy stannate and zinc (or other metal) stannate are currently used as flame-retardant additives. These include polyurethanes both solid and foam formulations, polyethylene particularly linear low density products, polypropylene, natural and synthetic rubber latex, polyamide, polystyrene, epoxies, neoprene, phenolics, EPDM/EVA blends, and particularly polyvinylchloride formulations. When used in concentrations of 1-30% more usually 5-20%, by weight on the weight of the polymer, these additives provide useful flame-retardant properties.
The following examples relate to intermediate zinc stannate products and blends which are shown to be superior to Flamtard H (commercial zinc hydroxy stannate) and Flamtard S (commercial zinc stannate) in certain respects:
Better critical oxygen index values (for both normal COI and elevated COI).
Electrical properties (improved volume resistivity).
Improved general fire performance (cone calorimeter data).
Reduction in smoke (NBS smoke chamber and cone calorimeter data).
It may be inferred that corresponding improvements would be shown by other divalent metal stannate products such as bismuth stannate products.